Fix prototype pollution in _.unset and _.omit (#6063)

Prevent prototype pollution on baseUnset function by:
- Blocking "__proto__" if not an own property
- Blocking "constructor.prototype" chains (except when starting at primitive root)
- Skipping non-string keys

See: https://github.com/lodash/lodash/security/advisories/GHSA-xxjr-mmjv-4gpg

_baseUnset.js

@@ -3,6 +3,12 @@ var castPath = require('./_castPath'),
     parent = require('./_parent'),
     toKey = require('./_toKey');
 
+/** Used for built-in method references. */
+var objectProto = Object.prototype;
+
+/** Used to check objects for own properties. */
+var hasOwnProperty = objectProto.hasOwnProperty;
+
 /**
  * The base implementation of `_.unset`.
  *
@@ -13,8 +19,47 @@ var castPath = require('./_castPath'),
  */
 function baseUnset(object, path) {
   path = castPath(path, object);
-  object = parent(object, path);
-  return object == null || delete object[toKey(last(path))];
+
+  // Prevent prototype pollution, see: https://github.com/lodash/lodash/security/advisories/GHSA-xxjr-mmjv-4gpg
+  var index = -1,
+      length = path.length;
+
+  if (!length) {
+    return true;
+  }
+
+  var isRootPrimitive = object == null || (typeof object !== 'object' && typeof object !== 'function');
+
+  while (++index < length) {
+    var key = path[index];
+
+    // skip non-string keys (e.g., Symbols, numbers)
+    if (typeof key !== 'string') {
+      continue;
+    }
+
+    // Always block "__proto__" anywhere in the path if it's not expected
+    if (key === '__proto__' && !hasOwnProperty.call(object, '__proto__')) {
+      return false;
+    }
+
+    // Block "constructor.prototype" chains
+    if (key === 'constructor' &&
+        (index + 1) < length &&
+        typeof path[index + 1] === 'string' &&
+        path[index + 1] === 'prototype') {
+
+      // Allow ONLY when the path starts at a primitive root, e.g., _.unset(0, 'constructor.prototype.a')
+      if (isRootPrimitive && index === 0) {
+        continue;
+      }
+
+      return false;
+    }
+  }
+
+  var obj = parent(object, path);
+  return obj == null || delete obj[toKey(last(path))];
 }
 
 module.exports = baseUnset;